Usage

uglifyjs[input files][options]

UglifyJS2 can take multiple input files. It's recommended that you pass the
input files first, then pass the options. UglifyJS will parse input files
in sequence and apply any compression options. The files are parsed in the
same global scope, that is, a reference from a file to some
variable/function declared in another file will be matched properly.

If you want to read from STDIN instead, pass a single dash instead of input
files.

The available options are:

--source-map Specify an output file where to generate source map.
[string]
--source-map-root The path to the original source to be included in the
source map. [string]
--source-map-url The path to the source map to be added in //#
sourceMappingURL. Defaults tothe value passed with--source-map. [string]--source-map-include-sources
Pass this flag if you want to include the content of
source files inthe source map as sourcesContent
property. [boolean]
--in-source-map Input source map, useful if you're compressing JS that was
generated fromsome other original code.
--screw-ie8 Pass this flag if you don't care about full compliancewith Internet Explorer 6-8 quirks (by default UglifyJS
will tryto be IE-proof). [boolean]
--expr Parse a single expression, rather than a program (for
parsing JSON) [boolean]
-p, --prefix Skip prefix for original filenames that appear in source
maps. For example -p 3 will drop 3 directories fromfile
names and ensure they are relative paths. You can also
specify -p relative, which will make UglifyJS figure out
itself the relative paths between original sources, the
source map andthe output file. [string]
-o, --output Output file (default STDOUT).
-b, --beautify Beautify output/specify output options. [string]
-m, --mangle Mangle names/pass mangler options. [string]
-r, --reserved Reserved names to exclude from mangling.
-c, --compress Enable compressor/pass compressor options. Pass options
like -c hoist_vars=false,if_return=false. Use -c with no
argument to use the default compression options. [string]
-d, --define Global definitions [string]
-e, --enclose Embed everything in a big function, with a configurable
parameter/argument list. [string]
--comments Preserve copyright comments in the output. By default this
works like Google Closure, keeping JSDoc-style comments
thatcontain"@license"or"@preserve". You can optionally
pass one ofthe following arguments to this flag:
- "all"to keep all comments
- a valid JS regexp (needs tostart with a slash) to keep
only comments that match.
Note that currently notall comments can be kept when
compression ison, because of dead code removal or
cascading statements into sequences. [string]
--preamble Preamble to prepend to the output. You can use this to
insert a comment, for example for licensing information.
This will not be parsed, butthe source map will adjust
forits presence.
--stats Display operations run time on STDERR. [boolean]--acorn Use Acorn for parsing. [boolean]--spidermonkey Assume input files are SpiderMonkey AST format (as JSON).
[boolean]
--self Build itself (UglifyJS2) as a library (implies--wrap=UglifyJS --export-all) [boolean]--wrap Embed everything in a big function, making the “exports”and “global” variables available. You need to pass an
argument to this option to specify thenamethat your
module will take when included in, say, a browser.
[string]
--export-all Only used when --wrap, this tells UglifyJS to add code to
automatically export all globals. [boolean]
--lint Display some scope warnings [boolean]
-v, --verbose Verbose [boolean]
-V, --version Print version number and exit. [boolean]

Source map options

UglifyJS2 can generate a source map file, which is highly useful for
debugging your compressed JavaScript. To get a source map, pass
--source-map output.js.map (full path to the file where you want the
source map dumped).

Additionally you might need --source-map-root to pass the URL where the
original files can be found. In case you are passing full paths to input
files to UglifyJS, you can use --prefix (-p) to specify the number of
directories to drop from the path prefix when declaring files in the source
map.

The above will compress and mangle file1.js and file2.js, will drop the
output in foo.min.js and the source map in foo.min.js.map. The source
mapping will refer to http://foo.com/src/js/file1.js and
http://foo.com/src/js/file2.js (in fact it will list http://foo.com/src
as the source map root, and the original files as js/file1.js and
js/file2.js).

Composed source map

When you're compressing JS code that was output by a compiler such as
CoffeeScript, mapping to the JS code won't be too helpful. Instead, you'd
like to map back to the original code (i.e. CoffeeScript). UglifyJS has an
option to take an input source map. Assuming you have a mapping from
CoffeeScript → compiled JS, UglifyJS can generate a map from CoffeeScript →
compressed JS by mapping every token in the compiled JS to its original
location.

To use this feature you need to pass --in-source-map
/path/to/input/source.map. Normally the input source map should also point
to the file containing the generated JS, so if that's correct you can omit
input files from the command line.

Mangler options

To enable the mangler you need to pass --mangle (-m). The following
(comma-separated) options are supported:

sort — to assign shorter names to most frequently used variables. This
saves a few hundred bytes on jQuery before gzip, but the output is
bigger after gzip (and seems to happen for other libraries I tried it
on) therefore it's not enabled by default.

eval — mangle names visible in scopes where eval or with are used
(disabled by default).

When mangling is enabled but you want to prevent certain names from being
mangled, you can declare those names with --reserved (-r) — pass a
comma-separated list of names. For example:

uglifyjs ... -m -r '$,require,exports'

to prevent the require, exports and $ names from being changed.

Compressor options

You need to pass --compress (-c) to enable the compressor. Optionally
you can pass a comma-separated list of options. Options are in the form
foo=bar, or just foo (the latter implies a boolean option that you want
to set true; it's effectively a shortcut for foo=true).

negate_iife -- negate "Immediately-Called Function Expressions"
where the return value is discarded, to avoid the parens that the
code generator would insert.

pure_getters -- the default is false. If you pass true for
this, UglifyJS will assume that object property access
(e.g. foo.bar or foo["bar"]) doesn't have any side effects.

pure_funcs -- default null. You can pass an array of names and
UglifyJS will assume that those functions do not produce side
effects. DANGER: will not check if the name is redefined in scope.
An example case here, for instance var q = Math.floor(a/b). If
variable q is not used elsewhere, UglifyJS will drop it, but will
still keep the Math.floor(a/b), not knowing what it does. You can
pass pure_funcs: [ 'Math.floor' ] to let it know that this
function won't produce any side effect, in which case the whole
statement would get discarded. The current implementation adds some
overhead (compression will be slower).

The unsafe option

It enables some transformations that might break code logic in certain
contrived cases, but should be fine for most code. You might want to try it
on your own code, it should reduce the minified size. Here's what happens
when this flag is on:

new Array(1, 2, 3) or Array(1, 2, 3) → [1, 2, 3 ]

new Object() → {}

String(exp) or exp.toString() → "" + exp

new Object/RegExp/Function/Error/Array (...) → we discard the new

typeof foo == "undefined" → foo === void 0

void 0 → undefined (if there is a variable named "undefined" in
scope; we do it because the variable name will be mangled, typically
reduced to a single character).

Conditional compilation

You can use the --define (-d) switch in order to declare global
variables that UglifyJS will assume to be constants (unless defined in
scope). For example if you pass --define DEBUG=false then, coupled with
dead code removal UglifyJS will discard the following from the output:

if (DEBUG) {
console.log("debug stuff");
}

UglifyJS will warn about the condition being always false and about dropping
unreachable code; for now there is no option to turn off only this specific
warning, you can pass warnings=false to turn off all warnings.

Another way of doing that is to declare your globals as constants in a
separate file and include it into the build. For example you can have a
build/defines.js file with the following:

const DEBUG = false;
const PRODUCTION = true;
// etc.

and build your code like this:

uglifyjs build/defines.js js/foo.js js/bar.js... -c

UglifyJS will notice the constants and, since they cannot be altered, it
will evaluate references to them to the value itself and drop unreachable
code as usual. The possible downside of this approach is that the build
will contain the const declarations.

Beautifier options

The code generator tries to output shortest code possible by default. In
case you want beautified output, pass --beautify (-b). Optionally you
can pass additional arguments that control the code output:

beautify (default true) -- whether to actually beautify the output.
Passing -b will set this to true, but you might need to pass -b even
when you want to generate minified code, in order to specify additional
arguments, so you can use -b beautify=false to override it.

inline-script (default false) -- escape the slash in occurrences of
</script in strings

width (default 80) -- only takes effect when beautification is on, this
specifies an (orientative) line width that the beautifier will try to
obey. It refers to the width of the line text (excluding indentation).
It doesn't work very well currently, but it does make the code generated
by UglifyJS more readable.

bracketize (default false) -- always insert brackets in if, for,
do, while or with statements, even if their body is a single
statement.

semicolons (default true) -- separate statements with semicolons. If
you pass false then whenever possible we will use a newline instead of a
semicolon, leading to more readable output of uglified code (size before
gzip could be smaller; size after gzip insignificantly larger).

preamble (default null) -- when passed it must be a string and
it will be prepended to the output literally. The source map will
adjust for this text. Can be used to insert a comment containing
licensing information, for example.

Keeping copyright notices or other comments

You can pass --comments to retain certain comments in the output. By
default it will keep JSDoc-style comments that contain "@preserve",
"@license" or "@cc_on" (conditional compilation for IE). You can pass
--comments all to keep all the comments, or a valid JavaScript regexp to
keep only comments that match this regexp. For example --comments
'/foo|bar/' will keep only comments that contain "foo" or "bar".

Note, however, that there might be situations where comments are lost. For
example:

Even though it has "@preserve", the comment will be lost because the inner
function g (which is the AST node to which the comment is attached to) is
discarded by the compressor as not referenced.

The safest comments where to place copyright information (or other info that
needs to be kept in the output) are comments attached to toplevel nodes.

Support for the SpiderMonkey AST

UglifyJS2 has its own abstract syntax tree format; for
practical reasons
we can't easily change to using the SpiderMonkey AST internally. However,
UglifyJS now has a converter which can import a SpiderMonkey AST.

For example Acorn is a super-fast parser that produces a
SpiderMonkey AST. It has a small CLI utility that parses one file and dumps
the AST in JSON on the standard output. To use UglifyJS to mangle and
compress that:

acorn file.js | uglifyjs --spidermonkey -m -c

The --spidermonkey option tells UglifyJS that all input files are not
JavaScript, but JS code described in SpiderMonkey AST in JSON. Therefore we
don't use our own parser in this case, but just transform that AST into our
internal AST.

Use Acorn for parsing

More for fun, I added the --acorn option which will use Acorn to do all
the parsing. If you pass this option, UglifyJS will require("acorn").

Acorn is really fast (e.g. 250ms instead of 380ms on some 650K code), but
converting the SpiderMonkey tree that Acorn produces takes another 150ms so
in total it's a bit more than just using UglifyJS's own parser.

API Reference

Assuming installation via NPM, you can load UglifyJS in your application
like this:

var UglifyJS = require("uglify-js");

It exports a lot of names, but I'll discuss here the basics that are needed
for parsing, mangling and compressing a piece of code. The sequence is (1)
parse, (2) compress, (3) mangle, (4) generate output code.

The simple way

There's a single toplevel function which combines all the steps. If you
don't need additional customization, you might want to go with minify.
Example:

After this, we have in toplevel a big AST containing all our files, with
each token having proper information about where it came from.

Scope information

UglifyJS contains a scope analyzer that you need to call manually before
compressing or mangling. Basically it augments various nodes in the AST
with information about where is a name defined, how many times is a name
referenced, if it is a global or not, if a function is using eval or the
with statement etc. I will discuss this some place else, for now what's
important to know is that you need to call the following before doing
anything with the tree:

Compression

The options can be missing. Available options are discussed above in
“Compressor options”. Defaults should lead to best compression in most
scripts.

The compressor is destructive, so don't rely that toplevel remains the
original tree.

Mangling

After compression it is a good idea to call again figure_out_scope (since
the compressor might drop unused variables / unreachable code and this might
change the number of identifiers or their position). Optionally, you can
call a trick that helps after Gzip (counting character frequency in
non-mangleable words). Example:

As usual, options is optional. The output stream accepts a lot of otions,
most of them documented above in section “Beautifier options”. The two
which we care about here are source_map and comments.

Keeping comments in the output

In order to keep certain comments in the output you need to pass the
comments option. Pass a RegExp or a function. If you pass a RegExp, only
those comments whose body matches the regexp will be kept. Note that body
means without the initial // or /*. If you pass a function, it will be
called for every comment in the tree and will receive two arguments: the
node that the comment is attached to, and the comment token itself.

The comment token has these properties:

type: "comment1" for single-line comments or "comment2" for multi-line
comments

value: the comment body

pos and endpos: the start/end positions (zero-based indexes) in the
original code where this comment appears

line and col: the line and column where this comment appears in the
original code

file — the file name of the original file

nlb — true if there was a newline before this comment in the original
code, or if this comment contains a newline.

Your function should return true to keep the comment, or a falsy value
otherwise.

Generating a source mapping

You need to pass the source_map argument when calling print. It needs
to be a SourceMap object (which is a thin wrapper on top of the
source-map library).

orig: the "original source map", handy when you compress generated JS
and want to map the minified output back to the original code where it
came from. It can be simply a string in JSON, or a JSON object containing
the original source map.